The long-term goal of this project is to increase the service life of dental resin composite restorations by obtaining a fundamental understanding of the development of secondary caries. Our more immediate goal is to test the hypothesis that oral biofilms contribute to the degradation of the margin, leading to more frequent instances of secondary caries. That might occur because composite materials select for a more cariogenic flora at the margin, or because products of bacterial metabolism contribute to the breakdown of the composite itself. Neither mechanism is mutually exclusive, so each question will be addressed in complementary clinical and laboratory studies, as described in the Specific Aims: 1.) Compare the bacterial species composition of biofilms collected from the enamel interface of sound amalgam restorations, sound composite restorations, composites with early secondary caries, and composites with frank secondary caries. Human Oral Microbe Identification Microarrays will be used to provide biofilm profiles incorporating 272 species. We will be testing the hypothesis that composite-enamel interfaces are colonized by distinctive bacterial species, compared to amalgam-enamel interfaces. Such ecological selection could account for an increase in virulence of the biofilm at the tooth-composite interface. Near-infrared optical computed tomography (OCT) will be used as an important adjunct to our clinical caries diagnosis, by detecting caries earlier than conventional methods, and confirming sound interfaces. 2.) Bridge our clinical and laboratory studies by optimizing a biofilm reactor system for growing multi-species oral biofilm microcosms at the composite-enamel interface of restorations placed in extracted teeth and coated with saliva. This system will provide the basis for testing the hypothesis that products of bacterial metabolism contribute to composite breakdown. Plaque samples from the clinical study will be used to establish microcosms corresponding to composite-enamel interfaces with no caries, early caries, or frank caries. The HOMIM system will be used to monitor the species composition of microcosms, and determine the conditions needed to reproduce the major species profiles of biofilms from patient samples. 3.) Use the Minnesota Artificial Mouth to incorporate load cycling into the biofilm reactor model. Composite restorations placed in extracted teeth will be subjected to repeated cycles of saliva coating, biofilm growth in the reactor, and loading in the artificial mouth. Teeth restored with composites that generate different levels of shrinkage stress will be exposed to the different types of microcosm, and also to sterile saliva medium alone, with or without loading. Both OCT and micro-CT imaging will be used to monitor the loss of minerals in the tooth tissues, while microhardness testing and Fourier-transform infrared spectrometry will be performed to assess changes in the composites. This in vitro model system will be used to test the hypothesis that defined bacterial microcosms and mechanical loading exert individual and combined effects on the material properties of composite restorations, degradation of the tooth-restoration margin and the time to restoration failure. PUBLIC HEALTH RELEVANCE: Replacing failed dental restorations takes up 70% of a dentist's effort and contributes $5 billion to health care costs in the US. Among the different types of restorations, composite restorations have been shown to have a higher failure rate than amalgam restorations, with the main cause of failure being secondary or recurrent caries. The knowledge gained from this and future projects will help guide the design of the next generation of dental composite materials, which are likely to require reduced shrinkage stress, improved bond strengths, as well as antibacterial and cariostatic capabilities.